1. Field of the Invention
The present invention relates generally to a GPS-Based attitude determination system(GPS-Based ADS) for determining attitude and heading(or azimuth) of a moving body, such as a vessel, by combined use of the Global Positioning System (GPS) and an inertial navigation system (INS) including inertial sensors. More particularly, the invention pertains to a technique used in a GPS-Based ADS for monitoring occurrences of operational failures of the system and diagnosing the cause of each operational failure.
2. Description of the Related Art
Typically, a conventionally known GPS-Based ADS configured to determine position, velocity, attitude and heading(or azimuth) of a moving body by using a plurality of GPS antennas includes multiple inertial sensors and a GPS receiver which outputs the position, velocity, attitude and heading of the moving body. The GPS-Based ADS thus configured can provide highly accurate and robust information on the position, velocity, attitude and heading of the moving body by integrating outputs of the GPS receiver and the inertial sensors. While the GPS-Based ADS is often provided with a plurality of GPS antennas, this is not absolutely required. As an alternative, the GPS-Based ADS may employ a single GPS antenna configuration, although the GPS-Based ADS so configured can not obtain attitude information from the GPS receiver. To cope with this problem, the GPS-Based ADS having the single GPS antenna configuration may be configured to calculate the attitude of the moving body from acceleration data output from accelerometers or to obtain attitude information from magnetic sensors. The single GPS antenna configuration has one drawback, however, in that the GPS-Based ADS can not provide full attitude of the moving body including roll, pitch and heading thereof. Nevertheless, there is a general tendency today to integrally incorporate a GPS receiver module, inertial sensors and an integration filter of the GPS-Based ADS in a radome-like housing to achieve compact design and low cost.
In the aforementioned kind of GPS-Based ADS employing a single-structured sensor portion including the GPS receiver module, the inertial sensors and the integration filter built in the radome-like housing, it is essential that the sensor portion be installed at a location where multipath disturbance or blockage of satellite signals would not occur and the sensor portion would not be subjected to severe vibrations or impact. Practically, however, it might not be possible to install the sensor portion at such an appropriate location. Additionally, it is impossible to make sure that the sensor portion is mounted at an appropriate installation site under appropriate conditions, because it is difficult to make an on-site judgment as to whether the selected installation site and mounting conditions are appropriate.
If the installation site or mounting conditions of the sensor portion are inappropriate, the GPS-Based ADS would produce an operational problem. It is, however, extremely difficult for a user (or an installer) to judge whether the problem is caused by the inappropriate installation site or mounting conditions or by a failure of the GPS-Based ADS itself. Accordingly, the user (installer) would have to devote considerable time and labor to the solution of this kind of problem. The conventional GPS-Based ADS provides only such information as the number of GPS satellites from which GPS signals are currently tracked, positions of the GPS satellites and dilution of precision (DOP) indicating GPS satellite geometry, and not any information useful for making a judgment on appropriateness of the installation site and mounting conditions of the sensor portion.
The prior art discloses some approaches directed to the solution of the aforementioned problem of the conventional GPS-Based ADS.
For example, Japanese Patent Application Publication No. 2000-269722 proposes a technique for detecting GPS signal blockage by an obstacle and warning a user of the occurrence of such blockage by audible or visible means based on a difference in the levels of signals received by a plurality of antennas. This technique requires multiple antennas for detecting signal blockage, wherein the number of the antennas must be more than two. In a configuration with only two antennas, GPS signal blockage on both antennas will occur if the two antennas and an obstacle are located in line. Thus, the technique of comparing received signal levels can not be applied to a two-antenna configuration. In addition, the technique of the Publication does not provide any information which can be used for making a judgment on appropriateness of an antenna installation site and mounting conditions.
Another example of the prior art is found in Japanese Patent Application Publication No. 1997-297171, which describes a technique for determining whether an antenna of a GPS receiver is located outdoors or indoors. According to the Publication, a judgment is made to determine whether the antenna of the GPS receiver is located outdoors or indoors based on the number of GPS satellites used for navigation solution, comparative levels of received satellite signals, appropriateness of satellite elevation angles and signal tracking conditions. The technique of this Publication is intended to determine whether an object being monitored (i.e., the GPS antenna) is located outdoors or indoors by use of a signal blockage phenomenon, and not to identify causes of a system failure related to the antenna installation site and/or mounting conditions.